The present invention relates to the fabrication of integrated circuits. More particularly, the invention provides a technique, including a method and apparatus, for forming improved tungsten (W) films having decreased resistivity.
Deposition of tungsten over a semiconductor substrate is a common step in the formation of some integrated circuit (IC) structures. For example, tungsten is commonly used to provide electrical contact to portions of a semiconductor substrate. These electrical contacts are usually provided through openings in an insulation layer, such as a silicon oxide layer, formed over the substrate. One method used to form such contacts includes the chemical vapor deposition (CVD) of tungsten to fill the opening after an initial layer of titanium nitride has been deposited in the opening. As another example, tungsten is sometimes used to form metal lines over a semiconductor substrate.
One CVD technique that has been employed to deposit tungsten films in the semiconductor industry uses tungsten hexafluoride (WF.sub.6) and a hydrogen reducing agent, e.g., H.sub.2, as precursor gases. This technique includes two main steps: nucleation and bulk deposition. The nucleation step grows a thin layer of tungsten which acts as a growth site for subsequent film. In addition to WF.sub.6 and H.sub.2, the process gas used in the nucleation step of this technique includes silane (SiH.sub.4), and may also include nitrogen (N.sub.2) and argon. A bulk deposition step then is used to form the tungsten film. The bulk deposition gas is a mixture containing WF.sub.6, H.sub.2, N.sub.2, and Ar.
As advances in integrated circuit technology lead to a scaling down of device dimensions and an increase in chip size and complexity, improved methods of depositing tungsten are continuously being sought. Research has been performed using diborane (B.sub.2 H.sub.6) and other hydrides of Group III or V in place of or in addition to H.sub.2 in the process gas for both the nucleation and bulk deposition stages of CVD tungsten deposition. Some of this research suggests that tungsten films deposited from a process gas that includes B.sub.2 H.sub.6 exhibit reduced resistivity as compared to tungsten films deposited from a process gas without B.sub.2 H.sub.6.
Despite this discovery, further improvements in the deposition of tungsten films are desirable.